Stainless steel sheets have heretofore been excellent in durability and formability, and have a metallic appearance. Accordingly, they have been widely used as exterior materials in various technological fields.
For example, in the field of the automobile industry, a stainless steel sheet is used as an exterior part in a molding subsequent to roll forming and in a pillar subsequent to press forming, because the exterior part made of a stainless steel sheet can give an artistic appearance in which the unique brightness and feeling of metallic mass that the stainless steel sheet has are utilized by coating the steel sheet with a transparent resin.
However, it is difficult to readily form a transparent resin coating in a satisfactory form on the surface of such a stainless steel sheet exterior part. For example, in the method of coating a stainless steel sheet with a molten resin by extrusion, a resin having a specific composition suited to the extrusion conditions must be used. Moreover, the steel sheet surface must be thickly coated with the resin. It is therefore impossible to thinly coat an exterior part having a complicated shape with the resin.
It has also been proposed to form a resin coating by painting in a molten resin. For example, Japanese Unexamined Patent Publication (Kokai) No. 62-174387 discloses a method of forming a fluoro resin coating excellent in weathering resistance, resistance to dirt and slipperiness on the surface of a stainless steel. In order to improve the adhesive strength of the fluoro resin with the stainless steel, the method of forming a resin coating comprises coating the surface of the stainless steel with a silane coupling agent having an amino group, in a coating amount of 1 to 500 g/m2. Moreover, Japanese Unexamined Patent Publication (Kokai) No. 63-260965 discloses a transparent resin composition for stainless steel in which 0.1 to 5% by weight of a terminal epoxy group-containing silane coupling agent and 0.5 to 10 parts by weight of an oxalanilide ultraviolet absorber are added to 100 parts by weight of a resin containing 60 to 99% by weight of a solid component of a solvent-soluble fluoro resin having a hydroxyl value of 20 to 250 and 1 to 40% by weight of a solid component of a curing agent.
However, in the resin coating method in which a silane coupling agent is used, as described above, expected action and effect cannot be obtained so long as a silane coupling agent having a special composition is not employed. Moreover, in these methods, forming a resin coating having a uniform thickness is difficult, and articles having the shape of final products must be coated with a resin and baked individually. As a result, productivity and efficiency are reduced.
Furthermore, coating the surface of a stainless steel sheet with a resin film in place of extruding a molten resin or forming a resin coating has also been proposed (for example, refer to Japanese Unexamined Patent Publication (Kokai) No. 2-234859). The method of using a resin film can realize the following: easy formation of a coating having a uniform thickness; provision of a product having an appearance of high quality; and continuous lamination of a resin film to a flat sheet of a stainless steel and subsequent forming such as roll forming and press forming. Accordingly, the method also has the advantage of increased productivity. However, the method of laminating a resin film to a stainless steel sheet using a pressure-sensitive adhesive and a heat-sensitive adhesive restricts the properties (such as adhesive strength) of the adhesive to be used. The method therefore cannot be advantageously used for specific applications. For example, a stainless steel article used as an automotive exterior part must pass various endurance tests. However, the adhesive subsequent to the test can no longer show an adhesive strength comparable to that prior to the test. Accordingly, the stainless steel article with a resin film cannot be used as an automotive exterior part without further processing.
In order to supplement such a low adhesive strength between the stainless steel article and the resin film, there is a common method comprising continuously coating the surface of the stainless steel article with an acrylic primer and baking the primer in the processing steps (of Japanese Unexamined Patent Publication (Kokai) No. 2-234859, if. necessary). However, since the acrylic primer has a poor light resistance, the primer tends to peel off at the interface between the stainless steel and the primer when the primer is exposed to light over a long period of time. In order to avoid such problems produced by the acrylic primer, a transparent resin film excellent in light transmission cannot be used on a stainless steel article. Presently, stainless steel articles are coated by extrusion of polyvinyl chloride (PVC) or a colored resin film having a toning level sufficient to substantially prevent transmission of light. Thus it would be an advantage to provide a stainless steel as an automotive exterior article on which a transparent decorative film is formed.